1. Field of the Invention
This invention relates in general to incorporation of an alternate second operating system in a computer, and more particularly, to a method of incorporating a second bootable operating system in a disk drive with a temporary address offset feature that allows the entire disk address space to be accessible in the alternate address mode.
2. Description of Related Art
Personal computer systems perform initial code loading (booting) by reading the operating system from a predefined address on a disk file to a random access memory (RAM) in the central processing unit (CPU) of the computer system. In many personal computer systems it is advantageous or desirable to have two separate operating systems (OS) which may be separately activated. The different operating systems may be needed to overcome incompatibilities involving, for example, input/output (I/O) device addresses, I/O busses, peripheral devices, the Basic Input/Output System (BIOS), and the operating systems themselves. In other personal computer systems, it may be advantageous to have a second OS to update system files that cannot be accessed by the first OS or to run diagnostic programs which may be hidden from or otherwise unavailable during normal operation of the first OS by the user.
The prior art provides methods for partitioning the hard disk drive attached to the personal computer system to provide a memory area for storage (installation) of a first OS and second memory area for storage of a second OS. Usually each OS assumes that its Initial Program Load (IPL) and partition tables are located at absolute sector zero on the hard disk drive. An offset value to the address location of a second OS may be provided in a modified second BIOS that allows the second OS to be booted as if loaded at the absolute sector zero. One disadvantage of this method is that changes to the computer BIOS and additional BIOS read only memory (ROM) are needed adding to the complexity and cost of the personal computer system.
Another disadvantage of this method is that accessibility of the disk drive address space is limited to the address space allocated to the OS that is operable at a given time. Although the limited addressability provides protection against accidental alterations to the alternate OS files, the ability to use the second OS to provide diagnostic support for the first OS is severely limited.
Therefore, there is a need for a method of changing the OS of a computer system from a first OS to a second OS without the need to redesign the computer BIOS and without limiting addressability to the entire disk drive address space.
It is an object of the present invention to disclose a method to change operating systems (OS) of a computer system from a first OS system to a second OS system by having an address offset feature on a disk drive to access the second OS system.
It is another object of the present invention to disclose a method to configure the address offset feature so that the offset address space wraps around so that the entire disk drive address space remains addressable in the address offset mode of operation.
It is a further object of the present invention to disclose a method wherein the computer system automatically reverts to the first OS system from the second OS system when the computer system is powered on.
These and other objects and advantages are attained in accordance with the principles of the present invention by an address offset feature in a disk drive to temporarily offset the drive address space to a reserved area on the disk drive in which an alternate OS system exists. Computer systems perform initial code loading (booting) by reading from a predefined address on a disk drive. In the present invention, a first OS system is stored in a user area having a location of the first logical block address (LBA) designated LBA 0. A second OS system, which may be hidden from the user, is stored in a reserved area having a first LBA designated LBA R immediately following the user area in which the first OS system is stored. In an address offset mode (AOM) of operation, an address offset by an amount xe2x88x92R is applied to the disk drive address space such that the start of the reserved area is changed from LBA R to LBA 0. A command to set the maximum address of the disk drive address space to the sum of the user area address length and the reserved area address length effectively wraps the reserved area space around so that the reserved area effectively ends at the beginning of the user area. This wrap around of the offset address space allows the entire disk drive address space to be addressable in the address offset mode (AOM).
On advantage of the present invention is the availability of switching between two operating systems, especially for providing diagnostic services.
The above, as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed written description.